The present invention relates to power plants, and pertains particularly to a pressurized fluidized bed combined gas and steam turbine power plant having a system for maintaining the operation of the compressor to keep the plant in operation at low plant loads.
Combined cycle power plants having both gas and steam turbines have recently come into use in the power generating industry to meet a need for more flexible and efficient power plants. The steam turbines are typically fueled by coal or oil through a traditional boiler. The gas turbines are typically fueled by a premium fuel, such as natural gas or a clean fuel oil through an internal combustor. These combined power plants are primarily to provide flexibility to meet changing load demands and are traditionally fueled by separate fuels.
Pressurized fluidized bed power plants have been developed to meet a need for more efficient power plants for converting fossil fuels to electrical power with minimal pollution. The circulation fluidized bed combustor features a mixture of granular limestone or other sorbent materials supported on a non-sifting grid. An upward flow of air passes through the grid lifting and fluidizing the material. This results in a turbulent mixture of the bed particles having the free flowing properties of a liquid and providing an environment for stable combustion.
Fuels introduced into the bed will burn effectively, and sulfur dioxide released by the burning is chemically captured by a calcined limestone. The mixture of solids which includes ash and calcined limestone is recirculated through the combustor until the particle size is reduced sufficiently for elutriation through the cyclones. The fluidized bed combustor also has the advantage of being able to burn a wide variety of fuels. The pressurization of the fluidized bed combustor further enhances its efficiency.
Pressurized fluidized bed systems have been developed in recent years to further improve the efficiency of the fluidized bed combustor and boiler. These systems are also able to provide the total combustion for combined cycle power plants. The combined cycle power plant normally results in a highly flexible and efficient power plant. The fluidized bed combustor provides the hot gasses to power the gas turbine and to generate the steam for the steam turbine.
The gas turbine powers the compressor that provides the pressurized air for pressurization of the system, combustion and fluidization in the combustor. When plant load is reduced, the gas flow and temperature from the PCFB combustor eventually declines to the point where that there is no longer enough power from the gas turbine to power the compressor. Operation of the compressor is essential to keeping the plant running. In order to keep the compressor operating, either auxiliary fuel must be fired in an external combustor or the generator must be motored (run like a motor) on power from the grid.
Accordingly, it is desirable that an improved power plant to overcome these problems of the prior art be available.